1. Yes. That's a postal regulation, but if it gets separated somewhere within USCIS you'd like it to trace back to you.
2. Any valid USCIS address is fine. Make sure that the PAY TO line is filled out as specified on the form. (I did get an RFE because I abbreviated something.)
3. As already mentioned, the address on the Money Order doesn't matter either way. If you have the PO Box on there, leave it on there.
Fill out the Memo line...at least with your A number or case number. If it gets separated, that's what they'll look for.
Regards,
Vicky's Mom